Why does herbivore sex matter? Sexual differences in utilization of Fucus vesiculosus by the isopod Idotea baltica

Optimal feeding is often constrained by predation avoidance, which may dire ct feeding to take place on more sheltered, but less profitable, Darts of t he habitat. Though constrained, feeding preferences and/or digestive capabi lities should coevolve with the energetic and nutritional needs of the anim al. These needs, as well as risk taking for these goals, may differ between the sexes. We studied sexual differences in the utilization of the brown a lga Fucus vesiculosus by the marine isopod Idotea baltica. In a feeding pre ference test with small pieces of alga both sexes prefer the apical. newly grown parts to the basal parts. However, males show stronger preference tha n females. When isopods are reared on the apical or basal part of the alga alone, sexes grow differently on these parts. Females gain weight equally w ell and their intermolt duration is the same on both parts of the alga, whi le males perform clearly worse on the basal than on the apical part. Despit e the better quality of the apical parts of the alga, in a grazing experime nt in a "semi-natural" environment with whole bushes of F. vesiculosus also the basal parts are consumed, and even preferred over apical parts by fema les. We suggest that feeding preferences are subordinate to microhabitat ch oice, and therefore predation avoidance constrains optimal feeding. We hypo thesize that the better ability of females than that of males to utilize th e basal parts of the alga for growth is a co-adaptation to their risk-avers e general reproductive strategy. We discuss the sexual differences in host plant utilization in the context of reproductive strategies of the sexes, e specially different risk sensitivity of foraging, and its potential consequ ences on plant-herbivore interaction.